Many modern integrated circuits generally include several layers of metallic or conductive wiring (often termed "runners") which are surrounded and covered by dielectrics, illustratively formed from silicon dioxide. The presence of gates and field oxides, together with the conformal properties of deposited silicon dioxide, tend to make dielectrics very bumpy or uneven. The unevenness of dielectrics makes the formation of additional levels of reliable conductors problematic. Consequently, it is desired to planarize or smooth either dielectric layers or conductor layers. One method for planarizing dielectrics and/or conductors is a chemical-mechanical polishing (CMP). CMP has become a widely used technique for the planarization of both dielectric and metallic layers due to the high degree of global planarity that CMP provides. During CMP processing, the wafer, having partially fabricated integrated circuits thereon, is polished upon a polishing wheel. The resulting upper surface, being either dielectric or metal, becomes highly planar and provides a suitable base for a substrate upon which further layers of conductors or dielectrics may be formed.
However, those concerned with development of CMP techniques have found that it is difficult to determine when to stop the grinding or polishing of the dielectric or metallic layer. Frequently, the wafer is polished for an initial period of time. Then the wafer is removed from the polishing wheel and the thickness of the upper layer is measured. If needed, further polishing is performed for an additional period of time (and if needed, the process is repeated) until the desired layer thickness is achieved. The polishing rate of both dielectric and metals depends on a large number of factors and the polishing rate is therefore somewhat is variable. The variability of the polishing rate complicates the problem of obtaining consistent layer thickness.
Those concerned with the development of integrated circuit processing have consistently sought reliable and effective methods for detecting an appropriate end point so that CMP may be terminated without unduly repetitive measurements.